This paper presents the state of development of oxygen sensors based on the electromotive force (emf) measurement at null current, using yttria stabilized zirconia as solid electrolyte for application in liquid lead-bismuth eutectic (LBE), which is envisaged as a nuclear coolant or as a spallation target in accelerator driven system (ADS) for nuclear waste transmutation. The assembly procedure, the calibration method, as well as the summary of the various validation tests undergone in both static and loop facilities are presented so as to define a real state of achievement and the basics needs for further studies. The sensors are efficient, accurate, rapid and reliable for research loops. However, the poor mechanical resistance as well as the effect of traces of impurities, promoting an increasing time-drift under certain conditions, are to be further studied to improve the sensor reliability for a nuclear use. The oxygen and chromium solubilities were reassessed in the process of the sensor testing, those relations are also given and discussed.
The control of the impurities, and of oxygen in particular, is of major interest for ensuring adequate and safe operation of lead alloys facilities from the viewpoint of the corrosion phenomenon : spallation target or coolant for hybride or fast reactors, currently under studies within the transmutation topic of the 1991 law on nuclear waste disposal. In addition, because of the very low oxygen solubility in the lead alloys, it is compulsory to avoid saturation in any parts of a defined system and in any operating conditions so as to avoid any plugging by lead oxides built-up (fuel assembly feet, …). For the oxygen control, the on-line monitoring of the dissolved oxygen content is required. Electrochemical sensors built with yttria stabilised zirconia have been developed and tested in various static facilities : BIP, JACOMEX glove box, COLIMESTA. The experimental results have been compared to the theoretical formulation, and a calibration method applied (search for the singular point defining the saturation temperature). The operating range is as follows : 280°C-550°C, 10-10-10 ppm (1ppm=10-4 weight%), for a 40% estimated accuracy. Service life is more than 1000 hours up to now. Reproducibility, time drift, time to response, and mechanical resistance are satisfactory. Based upon these results a first validation of these oxygen sensors is obtained in static conditions.
15The corrosion of an austenitic steel in liquid sodium containing 189 µg.g -1 of oxygen was 16 investigated at 650°C as a function of time (122, 250 and 500 h). The steel samples were 17 characterized by means of complementary techniques, namely scanning electron microscopy, X-18 ray diffraction, glow discharge optical emission spectroscopy and transmission electron 19 microscopy. The characterizations showed that a NaCrO 2 oxide scale forms at the steel surface. 20Under this oxide scale, iron and molybdenum rich M 6 C carbide particles together with NaCrO 2 in 21 the grain boundaries and cavities filled with sodium were observed. The stainless steel substrate 22 and / or the chromite scale were dissolved in parallel with the formation of chromite and carbides. 23Thermodynamic calculations showed that NaCrO 2 and M 6 C are equilibrium phases in such a 24 system. NaCrO 2 is formed by the reaction of chromium diffusing from the steel bulk with sodium 25 and dissolved oxygen (external selective oxidation). Mo segregates to the steel surface where it 26 reacts with Fe from the steel and C dissolved in liquid sodium. The dissolution of stainless steel 27 occurred since the liquid sodium bath is not saturated in the dissolving species (pure metals and 28 oxides such as NaCrO 2 , Na 4 FeO 3 ). As for the cavities, vacancies are created at the steel / NaCrO 2 29 interface by Cr oxidation, carburization and dissolution of the other elements present in the 30 stainless steel. The vacancies become supersaturated and this leads to the nucleation of the 31 cavities observed. Part of the vacancies created by Cr oxidation or steel dissolution is annihilated 32 at sinks like dislocations leading to the translation of the oxide / metal interface towards the metal 33 bulk. 34
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